IMO most chemical metals are shit and do not remain solid enough when hot.

I've never used the hardblok though, so not sure of it's makeup or how robust it is either, or whether it's a pour in type of deal, or a putty type stuff like most chemical metals.

But you will need to get it very clean down in there no matter what you put in.

Surely brazing or something like that would be a more permanent repair ? It would definitely adhere to the cast iron, that's for sure.